Photovoltaic cells having back side contacts are a particular type of cell having their two metal electrodes on the back side i.e. their side opposite the side which receives solar radiation.
Electrical connection pads are formed on each of said electrodes respectively for their connection to an electric circuit allowing collection of the photogenerated current.
This configuration of the cells is particularly advantageous for producing a module comprising a plurality of such cells.
It effectively simplifies assembly—all the interconnections being made on the same side—and increases the production yields of the modules.
In addition, it allows the cells to be brought closer to one another, the effect of which is a significant increase in the electric performance of the module.
At the current time, two interconnection modes of cells with back side contacts are chiefly employed.
One first possibility, as for standard photovoltaic cells, is to use copper ribbons soldered onto the pads of each of the back side metal electrodes respectively. Alternatively, metal connectors can be used.
Document U.S. 2011/0126878 proposes interconnecting the cells using copper ribbons previously shaped to reduce thermal mechanical stresses due to the difference in the thermal expansion coefficient between the material of the cells (generally silicon) and the copper.
One second possibility for cells known as MWT cells (Metal Wrap Through) or EWT cells (Emitter Wrap Through) is to solder or glue the cells onto a metallised film.
In general it is the protective backsheet, the chief functions of which are to ensure electric insulation and to protect the module against outside attack, which meets this additional support-giving role, a copper layer comprising the metal interconnect lines being arranged on the side of the protective backsheet in contact with the connection pads on the back side of the cells.
In this respect reference can be made to U.S. 2011/0067751 which describes such a locally metallised, protective backsheet.
One disadvantage of this technique is the high cost of such a metallised film, which is produced using processes similar to those used to form printed circuits.
In addition, this technique entails a significant loss of material since the formation of the conductive traces generally assumes that the metallised film is deposited on the entire side of the protective backsheet followed by chemical etching so as to maintain only those regions intended to form the traces.
Document U.S. Pat. No. 5,972,732 describes several variants of this technique.
According to one embodiment, the conductive traces are formed on a substrate in the form of a mesh in polymer material, said mesh being inserted between the protective backsheet and the cells. The assembly is then laminated with an encapsulating material which passes through the mesh openings to ensure the mechanical strength of the module.
In all cases these techniques have the drawback of requiring the positioning of an electrically insulating layer between the cells and the connectors or metal traces to prevent short-circuiting.
Said electrically insulating layer may be present on each cell, on the connector or on the metallised film.
Alternatively said insulating layer may be an independent layer inserted between each cell and the connector or metallised film.
Document WO 2012/059534 describes a particular example of such an electrically insulating layer, in the form of a glass fibre fabric. Said layer is placed between the cells and the connectors, the electrical connection being obtained through the fabric. This requires that the mesh openings of said fabric should be sized so that when the cells are connected to the connectors by soldering the molten fusible alloy is able to pass through the fabric.
Irrespective of the form of this insulating layer, the positioning thereof entails an additional step in the fabrication of the module.
It is therefore one objective of the invention to design means for interconnecting photovoltaic cells having back side contacts which overcome the aforementioned disadvantages.
In particular, it must be possible to obtain the interconnecting of the cells by applying a minimum number of steps and using the least possible number of separate components.